Like oil in the 20th century, water could well be the essential commodity on which the 21st century will turn.

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Human beings have depended on access to water since the earliest days of civilization, but with 7 billion people on the planet as of October 31, exponentially expanding urbanization and development are driving demand like never before.

Water use has been growing at more than twice the rate of population increase in the last century, said Kirsty Jenkinson of the World Resources Institute, a Washington think tank.

Water use is predicted to increase by 50 percent between 2007 and 2025 in developing countries and 18 percent in developed ones, with much of the increased use in the poorest countries with more and more people moving from rural areas to cities, Jenkinson said in a telephone interview.

Factor in the expected impacts of climate change this century -- more severe floods, droughts and shifts from past precipitation patterns -- that are likely to hit the poorest people first and worst "and we have a significant challenge on our hands," Jenkinson said.

Will there be enough water for everyone, especially if population continues to rise, as predicted, to 9 billion by mid-century?

"There's a lot of water on Earth, so we probably won't run out," said Rob Renner, executive director of the Colorado-based Water Research Foundation.

"The problem is that 97.5 percent of it is salty and ... of the 2.5 percent that's fresh, two-thirds of that is frozen. So there's not a lot of fresh water to deal with in the world."

Over a billion people lack access to clean drinking water, and over 2 billion live without adequate sanitation, leading to the deaths of 5 million people, mostly children, each year from preventable waterborne disease, Renner said.

Only 8 percent of the planet's fresh water supply goes to domestic use and about 70 percent is used for irrigation and 22 percent in industry, Jenkinson said.